1. Field of the Invention
This invention relates to apparatus for extrusion of plastic sheet material, and in particular it relates to an improvement in the construction of annular dies for the extrusion of tubular thermoplastic film.
2. Description of the Prior Art
Tubular film extrusion dies have two basic structural components: an outer die body having a cylindrical opening, and a solid structure fitted within the cylindrical opening to provide a thin annular gap at the outlet end of the die. The ultimate die would have an internal structure and outer die lip which are machined to be perfectly round and an internal structure which would float within the cylindrical opening and be positioned in the exact center of the opening by some invisible force which could not obstruct or impede the flow of the molten extrudate thru the die.
In actual practice, however, the internal structure must be supported by some interconnecting support member which is sufficiently strong to enable the structure to withstand the force exerted by the flowing molten extrudate against it. the melt must part when going around the support structure or "spider", the rejoin prior to extrusion from the die. However, where the two streams rejoin there is a molecular discontinuity in the molten resin which results in a weak and noticeably visible weld line in the extruded tubular sheet. Further, the film gauge in the vicinity of the weld line is less than that of the remainder of the film. This is because the flow of the molten polymer in that area is impeded by two factors:
(1) it must travel a greater distance to detour around the support structure and back again; and PA1 (2) the melt flowing past the support is brought into contact with a larger wall area than is the melt which follows the less circuitous route, thereby encountering increased resistance to flow.
In the past, the visibility of the line has been minimized in some types of annular dies (e.g. the spiral die design) by making the two streams overlap at the point of rejoining so that the weld line runs diagonally to the surface of the film, but the reduction in film thickness in the vicinity of the weld has been more difficult to eliminate.
Two other important considerations in the construction of extrusion dies have been the purging characteristics of the die (which is of special concern when color changes are to be made in the extrudate from time to time) and the ease of cleaning. Both of these are functions of the mechanical structure of the die and are frequently found to be serious drawbacks in the more complex die designs which have attempted to alleviate the difficulties related to the aforementioned weld-line concerns.